148 VISCOUS THEORY OF GLACIER MOTION. [1846. 



must continually gain upon the sides at the rate of 150 feet per 

 annum at least, consequently the limit of cohesion of an elastic 

 solid would soon be overpassed, and plasticity in the material 

 sufficient to explain the whole motion would inevitably be 

 admitted at last. Independently of this, it is evident, that were 

 such a flexure essential to the motion, the lines of crevasses 

 would be convex in the direction in which the glacier is moving 

 instead of towards its origin. 



Argument from the Equable Progression of Glaciers. The 

 equability of the motions of the various parts of a glacier, united 

 as I have shown them to be by intricate relations,* must, I 

 think, appear conclusive to every one capable of forming a just 

 opinion on the subject, that the relative movements of the 

 various parts of the glacier are due to the action of forces at 

 small distances and to the antagonism of molecular cohesions and 

 molecular strains, and not to the casual jumbling of a quantity 

 of rude fragments. To myself, I confess that this now appears 

 the strongest argument of all for considering the glacier as a 

 united mass like a river, in which there is a nice equilibrium 

 between the force of gravitation, acting by hydrostatic pressure, 

 and the molecular resistances of the semi-solid ; the degree of 

 regularity of the law which connects the partial movements is 

 wonderful, and I maintain that it is inexplicable except upon 

 the viscous theory. Thus (1.) The glacier moves continually, 

 summer and winter, day and night, and never by fits or starts ; 

 for if it does if gravitation overcomes mere friction, it occasions 

 a shock or avalanche ; (2.) Its mean annual motion is nearly 

 alike from year to year ; (3.) The relative velocities of points 

 widely distributed over the glacier (but exposed to similar influ- 

 ences of climate), change simultaneously in the same directions, 

 often in the same proportions ; thus " the variation of velocity 

 in the breadth of a glacier is proportional to the absolute velo- 

 city at the time of the ice under experiment." f (4.) The pro- 

 gression of velocity from the side to the centre is marked by 



* See 5 of this paper, pages 108 and 109. f Travels, p. 149. 



